RU2579932C2 - Regulated alternative current resistive heater - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: regulated AC resistive heater contains a two-pole device, connected in series between a power network phase and a heater, created by the AC capacitor which capacitance is determined according to the lower limit of the power regulation zone of the heater connected to the two-pole device outlet terminals, and by two back-to-back circuits shunting the capacitor, each of them is made by completely controlled semiconductor rectifiers with pick-off diodes: back-to-back connected two IGBT-transistors shunted by bypass diodes, with connected emitters and collectors connected to the capacitor; or transistor with a pick-off diode; or two completely controlled IGCT-thyristors. The shunt diodes switch on at the moments of the capacitor alternating voltage passage through zero, and switch off with phase delay before the alternating current passage through zero such, that the phase control of the heater power is ensured.

EFFECT: network voltage remains high-quality even at the proportional powers of the network transformer and heater, and that connected network consumers operate without interferences, no additional filtration of the network power voltage is necessary.

2 cl, 5 dwg

Description

1. The proposed device is designed to control heating. It is connected between the alternating current network (alternate current, ac) and the ohmic heater and provides smooth control of the power released in it.

Known devices used for the smooth regulation of ohmic heaters, powered by an alternating current electric network. They use pairs of counter-parallel connected thyristors, introduced into the cut between the network and the ohmic heater. The change in power is carried out in them by a smooth shift of the moments of inclusion of thyristors [1], pp.135-138. Such devices are manufactured by many enterprises (a series of RNTT devices for currents from 100 to 1000 A, [2]).

Ohmic heaters with them have a number of advantages, such as:

- smooth power control at the network frequency, not accompanied by low-frequency pulsations (flicker);

- long service life due to the lack of switchable mechanical contacts;

- low power loss and compactness;

- low cost.

However, along with these advantages, thyristor-controlled heaters have a significant defect: poor electromagnetic compatibility; strong distortion generated by the power supply network. To filter these distortions and ensure the normal operation of the electric network and the consumers connected to it, additional costs are required that are many times higher than the initial costs of the thyristor regulator itself. The additional systemic costs of filtering at the same time invalidate the merits of the thyristor regulator.

The problem to which the claimed technical solution is directed is to preserve the advantages of thyristor-controlled heaters and at the same time remove the problem of electromagnetic compatibility. When using the proposed device, the mains voltage remains high quality even with the proportional capacities of the mains transformer and heater (technical result). Connected network consumers operate without interference. Additional network filtering is not required.

2. The desired effect - the elimination of strong distortions of the mains voltage - is achieved by the fact that instead of bipolar circuits from on-board semi-controlled thyristors in the proposed device, bipolar circuits composed of AC voltage capacitors and fully controlled semiconductor valves shunting them are introduced into the cut between the AC network and the ohmic heater ( e.g. insulated gate bipolar transistor - IGBT, or insulated gate commutated thyristor - IGCT). These gates are closed at the moments when the alternating voltage of the capacitor passes through zero, while not dissipating energy and keeping the voltage curve continuous. Shunt valves are turned off with a phase delay before the alternating current passes through zero. Shutdown is possible due to the controllability of IGBT or IGCT. The voltage curve during shutdown is also continuous. Power control is carried out by changing the phase off of the shunt valves.

3. The following drawings are presented, characterizing the essence of the proposed technical solution - an adjustable ohmic AC heater.

Figure 1 presents a diagram of an adjustable ohmic alternating current heater with a series connection of shunt transistor branches.

Figure 2 presents a diagram of an adjustable ohmic alternating current heater with parallel connection of shunt transistor branches.

Figure 3 presents the equivalent circuit of an adjustable ohmic AC heater.

In figure 4. The results of mathematical modeling of the operation of an adjustable ohmic heater in the mode with the greatest distortion of the mains voltage are presented.

Figure 5 shows the characteristics of the adjustable ohmic heater plotted by points as a function of the control parameter alpha.

4. The device of the proposed technical solution is presented in figure 1-2.

The adjustable ohmic AC heater consists of a two-terminal 1 (regulating element) connected in series between the phase of the supply network 2 and an ohmic heater 3. The two-terminal 1 is formed by an alternating voltage capacitor (capacitor bank) 4 connected in parallel and counter-parallel valve circuits made of fully controllable gates.

In figure 1, the shunt capacitor 4 counter-parallel valve circuits are formed by the counter-series connection of two transistors (for example, IGBT) 5 and 6, shunted by the reverse diodes 7 and 8. The emitters of the transistors 5 and 6 are combined, and the collectors are connected to the capacitor 4. power of the heater 3 is carried out using a control system 9, which receives control signals and signals from sensors (voltage, current), generally designated 10. Other consumers 11 can be connected to the mains supply using mutation devices 12.

In figure 2, one of the two shunt capacitor 4 counter-parallel valve circuits is formed by a transistor 13 with a cut-off diode 14, the other a transistor 15 with a cut-off diode 16.

Instead of transistors, any other fully controllable valves (i.e., valves capable of both closing and opening by a control signal), for example IGCT (insulated gate commutated thyristor), can be used in the proposed device. When using IGCTs with reverse blocking capability in the circuit shown in FIG. 2, cut-off diodes 14 and 16 become unnecessary. It is also clear that if it is necessary to ensure operation at high voltages, the proposed device can be used in series with IGBT or IGCT.

5. Below are explanations of the operation of the proposed device.

The configuration of the capacitor and its controlled bypass valves shunting it is described and analyzed in [3], pp. 216-234. Such a two-terminal network acts according to the main harmonics of current and voltage as an adjustable capacitance, which is variable from the installed capacitance C of the capacitor 4 (valves do not close) upwards to an infinitely large value (valves are constantly closed). At intermediate durations of the intervals for closing the valves, intermediate values of equivalent capacity are carried out.

Bipolar “capacitor-shunt transistors” are dual to bipolar “reactor-series thyristors”, i.e. all the properties of one of them are obtained from the properties of the other by replacing in the following set of paired categories:

voltage ↔ current

resistance ровод conductivity,

inductance

emf source; current source,

shunt circuit ери serial circuit.

Thyristor-controlled reactors are shunt type elements, and they have long been widely used in electrical networks to regulate reactive power. Dual bipolar - transistor-controlled capacitors - are elements of the series type. Their use in electric networks is limited, since networks (at the level of distribution and consumer networks) are mainly voltage sources.

In the proposed device, the difficulties of using capacitance controlled by switching transistors are removed by the synergetic effect of the interaction of elements:

- the capacitor 4 of the device ensures the voltage continuity of the ohmic resistance of the heater 3 and the voltage continuity of the network inputs and thereby provides a lower level of higher harmonics. For electromagnetic compatibility, the high decay rate of higher harmonics achieved by continuity with increasing order of n is especially important. As is known, in asymptotics, this rate of decrease for continuous variables is quadratic ~ 1 / n ² ;

- the ohmic resistance of the heater 3 dampens the oscillatory circuit formed by the short-circuit reactance of the network 2 and the capacitor 4 of the device and thereby prevents the occurrence of transient vibrations when switching transistors.

The implementation of strong parametric damping of transient oscillations is easily confirmed when considering a simple equivalent circuit of Fig.3. The diagram indicates:

17 - switched branch of a two-terminal 1,

18 is a signal for setting the power Pz of the heater 3,

19 and 20 are voltage sensors of the network 2 and capacitor 4, respectively.

The diagram (figure 3) shows typical relative values of the parameters of the elements. Normalization is performed according to the rated power of the heater 3, therefore, its ohmic resistance R≈1. The capacity of the capacitor 4 (capacitor bank) is determined by the set lower limit of the power control zone Pz of the heater 3; for example, if Pz min≈18% is set, then the installed conductivity of the capacitor 4:

yО≈45%.

Transformers of voltage classes 10 / 0.4, 10 / 0.69, 6 / 0.4, 6 / 0.69 have a typical short-circuit reactance of 4.5%. Thus, in the worst case, when the power of the transformer is equal to the power of the heater (other loads are negligible), the parameter xs reaches the value

xs≈4.5%.

Thus, the impedance is

ρ = (xs / yO) ^1/2 ≈0.316.

This is much less than the ohmic resistance R, so the expected transients will be aperiodic.

6. The operation of the adjustable ohmic heater is illustrated by the oscillograms obtained in the mathematical model (figure 4). Charts are given in relative units. When normalizing per unit taken:

- circular frequency - ω = 1,

- the amplitude of the phase voltage of the network - Vsm = 1,

- nominal amplitude of the phase current - Ism = 1.

Figure 4 (upper figure) on the graphs are given:

isa is the consumed phase current,

vea is the capacitor voltage,

vsa - phase emf network

ven is the neutral bias voltage,

vab - line voltage of network buses,

isal is the first harmonic of current consumption.

The table of contents in Fig. 4 shows the parameters of the equivalent circuit (Fig. 3) and the parameters of the mode. Graphs taken at control angle

alpha = 20.0 °

at which the distortion of the mains voltage reaches a maximum

iskv≈1.859%.

This maximum is quite small; distortion introduced by an adjustable ohmic heater is much less than the limit values allowed by the standard for voltage quality of networks GOST R 54149.

For reference, the table of contents also gives the effective value of the distorting part of the current consumption.

efiski≈5.294%

and amplitudes of the most intense current harmonics with numbers 5, 7

hisa 5≈6.879%,

hisa 7≈2.423%.

Current distortions are not regulated by the standard; it is enough that these currents do not distort the mains voltage in an unacceptable way.

Figure 4 (middle and lower figures) shows the spectra of the mains voltage spvab (f) and the current consumption spisa (f). The spectra are given on two scales so that both the levels of the lowest of the higher harmonics and the levels of higher-frequency harmonics, up to the 39th, are distinguishable. Spectra are given as a percentage. The 5th harmonic is expected to be the largest.

spvab (5) ≈1.423%.

Further, harmonics decrease rather quickly; for the 29th harmonica

spbab (29) ≈0.142%,

Figure 5 shows the characteristics of an adjustable ohmic heater constructed by points. Characteristics are given in the function of the control parameter alpha, which varies in the range

0≤alpha≤90 °.

At zero alpha, the transistors do not close at all. Conducts only a capacitor. At the same time, minimum power is achieved. With the capacitor conductivity adopted in the example, the minimum power

min P = P (0) ≅17.4%.

With an increase in alpha, the power monotonously increases, reaching a nominal value at alpha = 90 °:

P (90 °) = 100%.

In addition to P (alpha) in figure 5, two more graphs are given

efis (alpha) - consumed mains current, effective value,%

Vemx (alpha) is the amplitude of the capacitor voltage,%.

The consumed current efis in the entire range of operation does not exceed the nominal.

An additional useful feature of the adjustable ohmic heater is a graph of the amplitude of the voltage of the capacitor Vemx (alpha). In the entire range of operation, this amplitude does not exceed unity in relative units, i.e. does not exceed the amplitude of the phase voltage. Thus, the voltage class of the transistors of the proposed circuit is determined by the phase mains voltage, in contrast to the prototype, in which at certain control angles not phase, but linear voltage is applied to the thyristors. With a supply voltage of ~ 690 V, transistors designed for a supply voltage of ~ 400 V can be used in the proposed device (i.e., transistors with V _CES = 1200 V instead of transistors with V _CES = 1700 V). The former have significantly lower power losses compared to the higher-voltage latter. Similarly, with a network voltage of ~ 400 V, transistors are used that are designed for a network voltage of ~ 230 V, with the same gain in power losses.

Figure 5 (middle figure) is a graph of the dependence of the distortion of the mains voltage iskv on the control angle alpha. Throughout the entire operating range, the voltage distortion is small; the maximum at alpha = 20 ° is only 1.859%.

In Fig. 5 (lower figure), for reference, graphs of harmonics currents with numbers 5.7 (hisa 5 and hisa 7,%), as well as the total harmonic current efiski (%) are given. As can be seen from the graphs, the total value of efiski is close to the sum (in quadratures) of the harmonics currents 5, 7. The contribution of high-frequency higher harmonics is insignificant.

The above graphs are built for a specific combination of parameters xs, yО, however, they fully characterize the proposed device. The parameter xs (short-circuit reactance) for medium-voltage transformers varies very little. Here, in specific applications, one should expect rather lighter conditions when the power of the network transformer S is higher than the power of the heater P _N (other consumers are present). In this case, only the fraction of the transformer dissipation P _N / S · xs will be included in the calculations, and the introduced distortions will be less than in the basic example. Some aggravation of conditions may occur when conditions are tightened along the lower boundary of the regulation zone, i.e. reducing the set minimum power. In this case, it is necessary to reduce the conductivity of the shunt capacitance, and as a result of this, the distortion of the mains voltage will increase. However, this increase starts at a very low level, and the distortion remains small and quite acceptable. For example, if the minimum power task is reduced to Pmin = 8.5%, the capacitor bank needs to be reduced to us≈0.3. The maximum voltage distortion increases to iskv≈2.62%, which is significantly lower than the standard tolerance.

The proposed device is an inexpensive effective means of regulating ohmic heaters, eliminating the problem of distortion of the mains voltage for the entire class of such devices.

7. Thus, with the above performance of the claimed device, the tasks are solved:

- The above advantages of thyristor-controlled heaters are preserved;

- at the same time, the problem of electromagnetic compatibility is removed.

8. Based on the foregoing, the task of achieving the claimed technical result, which consists in the fact that:

- during the operation of the proposed device, the mains voltage remains high quality even with the proportional capacities of the mains transformer and heater;

- connected network consumers operate without interference, i.e. additional network filtering is not required;

effectively resolved.

Information sources

1. Suker K. Power Electronics. Developer's Guide. Moscow, ed. Dodeka XXI, 2007.

2. The company "Electroproject". RNTT voltage regulators. www.eep.rn.

3. Hingorani N.G., Gyugi L. Understanding FACTS. Concepts and Technology of flexible AC transmission Systems. IEEE Press, NY, USA. IEEE Order No PC5713.

Claims (2)

1. Adjustable ohmic AC heater containing a two-terminal from semiconductor valves connected between the phase of the supply network and the heater, characterized in that the regulating two-terminal is composed of an alternating voltage capacitor and fully controllable semiconductor valves shunting it, wherein the shunt valves are closed when the alternating voltage passes capacitor through zero, and power control is carried out by changing the phase of the opening of the shunt valves. 2. The adjustable ohmic AC heater according to claim 1, characterized in that insulated gate bipolar transistor (IGBT) or insulated gate commutated thyristor (IGCT) are used as shunt valves.

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